1 Corinthians 9:17-19
17 If I were volunteering my services of my own free will, then the Lord would give me a special reward; but that is not the situation, for God has picked me out and given me this sacred trust, and I have no choice. 18 Under this circumstance, what is my pay? It is the special joy I get from preaching the Good News without expense to anyone, never demanding my rights.
19 And this has a real advantage: I am not bound to obey anyone just because he pays my salary; yet I have freely and happily become a servant of any and all so that I can win them to Christ.Read full chapter